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Paternity test percentages explained

When you receive your DNA paternity profiling test report, a conclusion will provide one of two statements. For example, if we tested an alleged father named John Smith and a child named Jane Smith, our paternity report might conclude:

  • This result provides (extremely/moderately) strong support in favour of paternity
    This means that John Smith is considered to be the father of Jane Smith because the analysis shows that they share a parentchild relationship;


  • This result excludes John Smith as the biological father of Jane Smith
    This means that John Smith cannot be the father of Jane Smith because the analysis shows that they do not share a parentchild relationship.


If you have requested the DNA technical data sheet to accompany your DNA profiling test report, you will see technical data for each person being tested.

What is a DNA profile?

AlphaBiolabs examines 25 short tandem repeat (STR) markers when comparing the DNA profiles of tested individuals. These STR markers are specific locations on a chromosome made up of sequences of repeated DNA, and each are highly variable in length between individuals. Each individual has two copies of each STR marker, known as alleles: one is inherited from the father and the other from the mother.

The two alleles observed at each STR marker are compared between the tested individuals. A full match of these alleles between individuals provides evidence of a relationship between them. Each allele is represented by two numbers. For example in the DNA profile below, if the alleged father displays numbers 13 and 17.3 for one specific genetic marker, the child will need to display either the 13 or the 17.3 on the same genetic marker to show it has been inherited from this father. There will need to be a match with all STR markers tested for an inclusion of paternity.

MotherChildAlleged fatherPI
STR marker Jane SmithJohn Smith 

What is the paternity index?

Each match receives a paternity index (PI) value (last column on the DNA profile). The PI is a statistical calculation that compares the DNA profiles obtained to an untested random individual within the general population, taking the ethnic background of test participants into account. In other words, it is a way of measuring the strength of a particular match based on the relative uniqueness of the allele observed. In the example above, Jane Smith and John Smith share allele 17.3 at marker D16S539. The paternity index for this match is 2.78.

However, if most people have allele 17.3 at this particular location, the PI might be lower. The fact that the two tested individuals match at D16S539 — could indeed be because John Smith is the biological father of the child — but it could also be because most people would match with this allele at this location.

Conversely, if very few people have allele 17.3 at this location because it’s rarer, the PI value would increase.

A PI value of 0.00 indicates that there is no match between the two profiles at this location.

What is the combined paternity index?

The combined paternity index (CPI) is a calculation (the product of all paternity indices) that helps formulate the probability of paternity. If the CPI is greater than 1000, there is a match between the alleged father and child (support in favour of paternity). A CPI value greater than 1000 means that the probability of paternity is over 99%.

If the CPI is a zero, it is a non-match between the alleged father and child. A zero means that the alleged father is excluded as being the biological father of the child.

Other factors also need to be considered. Natural mutations can sometimes complicate the results. There could be cases where the alleged father and child share many common alleles and the CPI value is not strong enough for a conclusive result. In all cases, AlphaBiolabs would recommend including the biological mother’s DNA in any paternity test as this can significantly strengthen the results. We do not charge any extra fee for including the mother’s sample.

What is the probability of paternity?

The probability of paternity (shown as a percentage) for your DNA paternity test is calculated from the CPI and indicates the chance of paternity based on the complete DNA test results. In other words, it is the statistical probability of John Smith being Jane Smith’s biological father and not any other man from the same ethnic group who may share a similar DNA profile by random chance. When the probability of paternity is 99.99% for example, this means that John Smith is 99.99% more likely to be the biological father of Jane Smith than another man chosen at random from the same ethnic group.

Why is probability of paternity never 100%?

High probabilities of 99% and above are commonly seen in DNA paternity testing, but never 100%. This is because results are based on statistical calculations. A result of 100% would only be possible if AlphaBiolabs tested every male of the same ethnicity as the biological father. This is why the DNA profiling test report uses the wording ‘provides (extremely/moderately) strong support in favour of paternity’. A probability over 99% means there is relatively no chance any other man, other than the tested man, is the father of the child (unless the biological father is a close relative of the alleged father; such information should be divulged when ordering your paternity test).

Further information on paternity testing, the DNA process and other FAQs can be found on our Learning Centre.